In DS spread spectrum communications such as in code division multiple access (CDMA) systems, a signal for transmission is spread in the frequency domain. Thus the same information is transmitted over a different (larger) bandwidth, affording some level of diversity in the frequency domain. The benefits of such a communications method are that the transmitted signal is more resilient to band limited interference and frequency selective fading.
The spreading of a signal is achieved by modulating each symbol, or each set of consecutive symbols, of the transmission signal by a sequence or code that varies at a faster rate than that of the transmission signal. Typically in DS-CDMA systems the spreading waveforms are selected from a long pseudo random sequence, or if short enough they can be chosen according to some optimality criteria. Whilst longer sequences are desirable due to their superior correlation properties, shorter sequences are typically employed due to the resulting ease of implementation of both transmitter and receiver algorithms and associated hardware. As an example of a method for the design of short sequences, but not restricted to this method, the sequences may be chosen such that they exhibit good auto-correlation properties or such that pairs of sequences exhibit favourable cross-correlation properties. This typically requires a search over all possible sequences of the desired length to select the sequences with the desired attributes. For each new spreading length required, a new search is required for sequences that satisfy the desired optimality criteria.
However, this approach has the disadvantage(s) that, since good spreading waveform sequences require a search over all possible sequences of a desired length to select the sequences with the desired attributes, and since for each new spreading length required a new search is required for sequences that satisfy the desired optimality criteria, longer sequence lengths are inefficient to design.
A need therefore exists for efficient spread spectrum code construction wherein the abovementioned disadvantage(s) may be alleviated.